Disc lasers have a laser-active medium (a gain medium) of a small thickness (laser disc), which can be cooled well. As a result, the concept of the disc laser is suitable for high laser powers in the multi-kilowatt range. Because of the small thickness of the gain medium, however, little pumping light is absorbed in a single pass through the laser-active medium, which without the provision of suitable measures would have the consequence of a low efficiency of the laser system when pumping the laser-active medium. Multiple passes of the pumping radiation through the laser-active medium are generally warranted to obtain a minimum energy or minimum laser power required for satisfying the laser condition in the laser-active medium. In the case of such multiple passes, the pumping light beam is repeatedly focused onto the laser-active medium, ideally a 1:1 imaging of the pumping spot being produced in successive focusing steps. The repeated imaging via the concave mirror leads to an impairment of the collimation in the parallel beam path, so that the collimated beam diameter increases and the power coupled into the laser-active medium decreases, which limits the number of multiple passes.
EP 1 252 687 B1 and the dissertation “Pumpoptiken and Resonatoren für Scheibenlaser” [Pumping Optics and Resonators for Disc Lasers], S. Erhard, University of Stuttgart, 2002, ISBN 3-8316-0173-9, discloses a pumping arrangement in which the pumping light beam is focused onto the laser-active medium with the aid of a parabolic mirror. The multiple passes are achieved by the pumping light beam being deflected repeatedly with the aid of deflecting devices, for example prisms, between different reflecting regions that lie in different sectors of a circle in one and the same annular region of the parabolic mirror.
The dissertation “Scheibenlaser mit Kilowatt-Dauerstrichleistung” [Disc Laser with Kilowatt Continuous-Wave Power], C. Stewen, University of Stuttgart, 2002, ISBN 3-89675-763, discloses using multiple annular regions with reflecting regions on the parabolic mirror for the imaging by using multiple deflecting units, the individual rings being arranged radially in a segment of the parabolic mirror.
DE 10 2011 004 204 A1 discloses a pumping light arrangement having a reflecting surface that may have two, three or more annular regions with multiple reflecting regions. A deflection of the pumping light beam within a respective annular region takes place in the azimuthal direction.
Greater distances between the reflecting regions and the central axis of the reflecting surface, can yield greater imaging errors. In particular, for the case where three or more annular regions are used to deflect the pumping light beam or focus the pumping light beam onto the laser-active medium, at least the radially outermost annular region can beat a distance from the central axis of the reflecting surface that is so great that the imaging errors increase greatly, so that there is a noticeable expansion of the pumping light beam, which may possibly lead to so-called clipping effects (cutting-off losses) at deflecting devices, for example in the form of prisms.
If a pumping light source of a high beam quality is used, the beam diameter with which the pumping light beam coming from the pumping light source impinges on the reflecting surface is comparatively small. Therefore, the reflecting surface can be made to extend in the radial direction, thereby reducing or minimizing the problematic matter of beam expansion. However, pumping light sources of a high beam quality, for example diode lasers, are expensive. In particular, in the case of high pumping powers, however, pumping light sources that are as inexpensive as possible should be used, with which the aforementioned effects can occur.